- December 7, 2017 at 12:17 pm #48105Larry GibsonParticipant
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Average Velocity, ES and SD Variation
I was testing a new (to me) NOE bullet in my M39 Finn Mosin-Nagant 7.62x54R rifle I use in CBA Military Rifle matches so I thought, since I was shooting five 10 shot test strings of the same load, I would chronograph each 10 shot test string to not the subtle differences in the average velocity, ES and SDs of the same load. Since the CBA matches I attend are for group and for score (using the 100 and 200 yard reduced 600 yard NRA HP target) I shot three of the test strings at 100 yards to also test for accuracy and to confirm the 100 yard zero. I then would shoot the two remaining test strings at 200 yards to confirm the zero.
I had cast the NOE bullets using a 311-205 four cavity aluminum mould out of Lyman #2 alloy. The bullets were very uniform with few rejections during visual inspection and weight sorting. The bullets dropped just over .313 on the bands and .302+ on the nose. I seated and crimped the Hornady GCs and lubed them with 2500+ in a Lyman 450 with .314 H&I die. Fully dressed they weighed in right at 200 grains. I loaded them in NS’d Norma cases over 28.5 gr of milsurp 4895 with a 1 gr dacron filler. WLR primers were used. That is the match load I use with the Lyman 314299 which has won me two state championships and numerous matches. I was hoping the NOE bullet would shoot as well as the Lyman and it did.
Many times we see posted on forum a velocity given as XXXX fps with many, if not most, not understanding that chronographed figure is not an absolute. There always will be some variance in the average fps, the ES and the SD of any test of a lot of ammunition. What the variance can be is not only dependent on the quality of the ammunition but also on the number of shots in the test strings. I consider 3 shot tests as only giving an idea of what may be expected. A 5 shot test gives a better idea but still leaves considerable room for error. The 10 shot test is standard but even that gives an optimistic average, ES and SD fps.
The five 10 shot results;
Average fps, ES fps, SD fps
1813, 34, 12
1817, 25, 7
1812, 39, 10
1826, 42, 13
1821, 41, 13
We see here a total difference in average velocity between each test string of only 13 fps. The ES varied 22 fps and the SD varied, between strings, 6 fps. All of which indicates a very consistent load even considering the differences. However, to get a much better understanding lets look at the figures for the entire 50 shot test.
For the entire 50 shot test the average velocity was 1818 fps. The ES was 52 fps and the SD was 17 fps. Some would not consider those figures to be good yet given the winning record of the load it has proven to be a very good load. Bottom line is to actually have the best idea of a loads potential it pays to test a sufficiently large sample. This is why I don’t give much credence to less than a 10 shot test and really prefer at least three 10 shot back to back tests to confirm probable ballistics and accuracy potentential.
- December 7, 2017 at 1:18 pm #48110Rattlesnake CharlieParticipant
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Great post Larry. Thanks for taking the time to provide such detail. Your explanation on interpretation of data should open a few eyes. Especially on how many might consider your chrono data to indicate a load that should not perform as well as it does. And, the volume of data in a sample … yup. I am disappointed in current gun rag writers who only publish the results of a few three shot groups. Never hear of those that don’t make MOA, and we KNOW they happen. OK, I’ll reign it in.
- December 8, 2017 at 10:16 am #48125GoodsteelKeymaster
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Absolutely excellent post Larry. I agree wholeheartedly, and we need to understand that taking a significant sample size is paramount to having real results.
I have a question though.
How is the information you gathered with the chronograph applied to your shooting in the real world? IE: you describe very well how to gather that information. Why are you gathering it in the first place? What are you looking for, and what is the effect downrange?
I’ve heard a lot of opinions on this, but I would like very much to hear your process and what you do with this information once gathered.
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